1. Field of the Invention
The present invention generally relates to electronic devices for displaying text and graphics, and, more particularly, to thin-film electroluminescent panels.
2. Description of the Related Art
Electronic displays are used to display the output data from digital computers and other data generating devices, such as television systems, communications systems, and the like. By far, the most well-known and most widely used display device is the cathode ray tube (CRT) which is found in television sets, computer display monitors, and countless other devices in which textual information, graphic information, and/or video images are to be displayed. However, because of the relatively large sizes and weights of CRT's and the electrical power required to drive them, there has been extensive research and development directed towards replacement devices that provide the same or similar functions in a so-called "flat panel" device. Such devices include plasma displays, light-emitting diode displays, liquid crystal displays, and electroluminescent displays. This application is concerned with electroluminescent displays, and, primarily with thin-film electroluminescent displays.
A typical thin-film electroluminescent (TFEL) display panel comprises a matrix-addressed panel of a thin-film phosphor in a thin-film dielectric sandwich. The thin-film phosphor emits light when a large enough electric field is applied across it. For example, an electric field having a magnitude on the order of 2 megavolts per centimeter may be required to cause the phosphor to emit light. The electric field typically is provided by an electrode matrix that comprises a plurality of row electrodes and a plurality of orthogonally positioned column electrodes. The intersections of the row electrodes with the column electrodes define pixel cells. The pixel cells comprise the pixels of the TFEL display. When a voltage having a sufficient magnitude is applied between a row electrode and a column electrode, the phosphor of the pixel cell at the intersection will emit light. The magnitude of the voltage required to cause the phosphor to emit light is the threshold voltage.
In operation, a write voltage pulse is applied to the row electrodes, one row at a time (e.g., row one, followed by row two, and so forth). The write voltage pulse applied to the "addressed" row electrode (e.g., the first row) is below the threshold and is thus insufficient by itself to cause the phosphors of the first row to emit light. At the same time that the write voltage pulse is applied to the selected row electrode, a modulation voltage pulse is applied to each column electrode. If the pixel cell at the intersection of the addressed row and a column is to emit light, the modulation voltage pulse applied to the column is selected to be sufficient, when added to the write voltage pulse applied to the row, to be above the threshold voltage for the phosphor so that the pixel cell emits light. On the other hand, if a pixel cell is to remain off, the corresponding column modulation voltage is selected to be zero volts or some other voltage that, when added to the write voltage pulse applied to the row, is below the threshold voltage of the phosphor. After the first row has been written, the write voltage pulse is applied to the next row (e.g., row two), and a modulation voltage pulse is applied to each column to cause the phosphors of selected pixel cells in the second row to emit light. The sequence is repeated for each row until an entire frame has been written. In other words, the pixel cells in each of the rows will have been selectively caused to emit light or remain dark.
The phosphors of the TFEL panel typically comprise zinc sulfur manganese, or the like, sandwiched between two dielectric insulating layers. The sandwich structure requires an applied voltage that changes polarity in order to cause the phosphor to emit light. Basically, each of the dielectric sandwiches defining each pixel cell acts as a capacitor having the two electrodes as its plates and the phosphor as part of the dielectric. Thus, although the phosphor will emit light only for a relatively short amount of time after the threshold voltage is applied, the capacitor will remain charged. The voltage charge across a pixel cell will oppose the next application of voltage across the cell and prevent the threshold voltage of the cell from being reached. In order to be able to cause the pixel cell to emit light again, a voltage pulse of opposite polarity to the write voltage pulse is applied to the pixel cell to discharge the capacitance. This opposite polarity voltage pulse, called a refresh voltage pulse, is applied to the pixel cell once for each frame. In many TFEL panel systems, the timing of the refresh voltage pulse is such that two light pulses are emitted during each frame of the data that is displayed on the panel, once when the write voltage pulse is applied and once when the refresh voltage pulse is applied. For example, the refresh voltage pulse is typically applied to the entire panel after the last row in the frame is written so that all of the pixel cells are refreshed at the same time. It should be understood that only the cells to which the write voltage pulse was applied will emit light during the refresh voltage pulse.
It has been found that the foregoing system for refreshing TFEL panels has a problem with image latency. In other words, although a pixel cell has not been activated by the application of a voltage above the threshold voltage, the pixel cell will emit light. It is believed that the latent image problem arises after a pixel cell has been activated for a substantial amount of time because of the buildup of electrical charge on the cell walls. The built-up charge occurs because the voltage pulses applied to the pixel cell are not symmetrical with respect to time. In other words, for some of the pixel cells, a voltage pulse of one polarity (e.g., the write voltage pulse) is applied to the cell a relatively long amount of time before the opposite voltage pulse (e.g., the refresh voltage pulse) is applied. After the refresh voltage pulse, there is a relatively short amount of time before the write voltage pulse is again applied. The built-up charge on the cell wall effectively lowers the threshold voltage required to activate the pixel cell such that the write voltage pulse applied to the row electrode of the cell alone will activate the cell irrespective of the modulation voltage pulse applied to the column electrode of the cell. For other cells, the relative time durations may be reversed such that the refresh voltage pulse is applied a relatively long amount of time prior to the write voltage pulse. Thus, after the TFEL panel has been used for a substantial amount of time, the panel will become less and less useful for its intended purpose.
There have been attempted solutions for reducing or eliminating the latent image problem. For example, one solution has been to provide a symmetric drive system. In an exemplary symmetric drive system, a high voltage push-pull circuit is used to apply the write voltage pulse to the rows and the modulation voltage pulse to the columns. During one frame, the applied voltage pulses are of one polarity so that the cell voltages are applied in a first direction. In alternating frames, the polarities of the write voltage pulse and the modulation voltage pulse are reversed so that the cells are charged in the opposite direction. The alternating application of the opposite voltage pulses across the pixel cells eliminates the need for the refresh voltage pulse and has the effect of causing the voltages applied to the cells to be symmetrical in time and voltage. This technique has the disadvantage that only one light pulse is emitted per frame so that the panel only provides half the brightness. Furthermore, the electronic circuitry is more complicated because each of the high voltage electrode drivers has to drive the panel with two voltage polarities rather than just one.